A massive eradication effort wiped out screwworms in the U.S. 35 years ago—but then they reappeared.

The stray dog came with bad news. This week, the U.S. Department of Agriculture announced that a dog near Homestead, Florida—a city 15 miles north of the Florida Keys—was found with wounds infested with screwworms, the much dreaded flesh-eating pest.

If you’re not familiar with screwworm, it’s because the U.S. poured millions of dollars into eradicating them back in 1982. But last fall, it reemerged in the Florida Keys, catching almost everyone by surprise. Wildlife biologists eventually found several deer on the archipelago with the parasite. Screwworms lay eggs in open wounds, burrowing into the flesh of pets and occasionally even humans. Livestock, historically, was the big economic concern. Florida still sends hundreds thousands of young calves to herds around the country each year, so a screwworm infestation could do some real damage.

“The screwworm is a potentially devastating animal disease that sends shivers down every rancher’s spine,” said Florida’s Commission of Agriculture Adam Putnam, in a statement that accompanied the official declaration of agricultural emergency last October.

The Keys infestation was bad, but at least it was somewhat isolated on the archipelago. Officials set up an animal health checkpoint at mile 106 on U.S. Highway 1, the main road that leads from the mainland to the Keys. The checkpoint would scan animals leaving the Keys—usually pets traveling with their owners—for infestation with screwworm.

It’s not clear exactly how that stray dog got infested or where it had been before it was found in Homestead. The USDA heard about the animal from a vet in the area. It has since be treated. “It’s a very treatable condition if caught early,” says USDA veterinary medical officer Robert Dickens. “The dog is doing really well.”

Individual animals can get anti-parasite drugs. But the best large-scale weapon against screwworms are sterile screwworms, deliberately sterilized using X-rays in a factory. Release enough of them and they will prevent any of the non-sterile ones from finding a mate.

This was the strategy that eradicated screwworms from the U.S. 30 years ago, and this is the strategy USDA has been using to get screwworms out of the Keys again. The USDA has released 80 million sterile screwworms across 25 sites in the Keys, and now will add Homestead to the list of release sites. On Friday, state and federal teams released sterile screwworms at Homestead, and they will continue doing so twice a week for the next six to nine weeks.

It’s still unclear where the screwworms came from. After the U.S. eradicated the pests, it partnered with countries in South and Central America, releasing sterile screwworms further and further south until they reached one of the narrowest parts of the continent, the Darien Gap in Panama. Here, millions of sterile screwworms are still dumped by the airplane-load to form an invisible but permanent sterile insect barrier.

Perhaps someone or something brought it to the Keys from further south. Perhaps it came from Cuba, Haiti, or the Dominican Republican, which have not eradicated screwworms and are just short expanse of ocean away. Wherever it came from, it eventually reached a stray dog in Florida.